What are the advantages of separating FK and deform bones in an IK/FK rig?
One advantage is that you can see the FK bones actual positions without adjusting the IK/FK switch. This might be useful when blending halfway between IK and FK in order to transition from one to the other. When going from IK to FK you can set your FK positions to their destination position and then adjust the switch and see the deform transition state. You'll still be able to adjust the destination FK position while seeing the resulting transition state without having to go back and forth between them.